JP-2012-177303A describes that a controller relates to a fuel injector injecting fuel by a lift-up (valve-opening operation) of a valve body according to an electromagnetic attractive force generated by an energization of a coil. An opening time point of the valve body and an opening time period are controlled by controlling an energization start time point of the coil and an energization time period of the coil, and then an injection start time point and an injection amount are controlled.
As shown in FIGS. 16A to 16E, the controller executes an increase control to increase a coil current to a first target value I1 by a boost voltage that is boosted from a battery voltage and is applied to a coil. Therefore, the valve body starts to open at a time point t1 that an electromagnetic attractive force reaches a required valve-opening force Fa. In this case, a current for holding the valve body at a position corresponding to a maximum-lift position is less than the first target value. Specifically, when the electromagnetic attractive force is increased, the electromagnetic attractive force is affected by inductance due to a large variation in a magnetic field. When the electromagnetic attractive force is held to a specified value, the electromagnetic attractive force is not affected by inductance.
Thus, at a time point t20 that the coil current reaches the first target value I1, a duty control corresponding to a current-stabilizing control controls a voltage to be applied to the coil to decrease the coil current so that the coil current becomes a second target value I2 that is less than the first target value I1.
FIG. 16D is a graph showing a Ti-q property line representing a relationship between an energization time period Ti of the coil and an injection amount q in a case where the valve body is opened. A flow-throttling degree at an injecting port becomes greater than the flow-throttling degree at a seat surface of the valve body, in a normal injection area in which a lift value is greater than or equal to a predetermined value. The normal injection area corresponds to an injecting-port throttle area B2. The injection amount is determined according to a throttling of a flow at the injecting port. The flow-throttling degree at the seat surface becomes greater than the flow-throttling degree at the injecting port, in a small injection area in which the lift value is less than the predetermined value. The small injection area corresponds to a seat throttle area B1. Therefore, the injection amount is determined according to the throttling of the flow at the seat surface.
The higher a temperature of the coil becomes, the greater a resistance of the coil becomes. In this case, as dotted lines shown in FIGS. 16A and 16B, a time period from a time point t10 that a voltage starts to be applied to the coil to a time point t20 that the coil current reaches the first target value I1 becomes longer. Therefore, an increasing slope of the electromagnetic attractive force becomes gradual as shown in FIG. 16C, a valve-opening start time point t1 is delayed, and a valve-opening time period t1 to t5 becomes shorter.
In other words, when a coil temperature varies, an increasing slope of the current varies. Therefore, the increasing slope of the electromagnetic attractive force varies, and the Ti-q property line varies. When an injection state is controlled to achieve a request injection start time point and a request injection amount, a robustness of a control of the injection state is deteriorated relative to a variation in the coil temperature.
When a multi-injection in which fuel is divided to be injected for multiple times in a single combustion cycle is executed, it is required that a small amount of fuel is accurately injected. In this case, since an affect of a time lag of the injection start time point with respect to a differential amount of the injection amount is increased, an accuracy of the injection amount becomes remarkably worse due to the variation in the coil temperature.